Method for controlling the position of a permanent magnetically supported rotating component

ABSTRACT

Since disruptive forces can be continuously exerted upon a rotor during a pulsatile flow through a pump with a magnetically mounted rotor (permanent magnets and addition control current coils), it is necessary to adjust position i.e. modify axial rotor position very quickly. The control current should only result in small amounts of losses. According to the invention, the current is pulse-width modulated by the control current coils by means of a set value predetermined by a controller which is arranged downstream from a position sensing system. If the set value is high, it is switched to a higher voltage level and the real value of the position sensing system is stored for a defined period of time, respectively beginning with the switching flank of the control current, and the position sensing system is disconnected during said period of time.

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling the position of apermanent magnetically supported rotating component, e.g. of the rotorof a synchronous motor without brushes, by means of a positiondetermination of this component by means of a position sensory analysisand additional control current coils, influencing the magnetic field ofthe permanent magnetic support and which current value is determined bythe position of the component. The synchronous motor can for exampleserve as a drive for an axial fluid delivery pump.

Multiphase fluids, e.g. emulsions and dispersions with a low stabilitycan easily reach into instable areas during the delivery incorresponding delivery systems.

An especially sensitive fluid is blood. Blood is hermetically shieldedin the natural recirculation system from the environment, so that noforeign interferences are acting upon it. If, however, the necessityexists, to substitute the heart by an artificial blood pump or tosupport the recirculation by an additional heart pump, reactions of theblood with the technical system are produced. The blood is subjected,then, easily to the haemolysis or the formation of thrombus with thecorresponding disadvantageous effects for the patient. Therefore,recently large efforts were made, to form fluid delivery pumps in such away, that the blood or other sensitive fluids are subjected to thelowest possible mechanical influences. One possibility for this is themagnetic support of the rotating element of a pump drive. The advantageof the magnetic support is not only, that no components mechanically ina frictional way are present any more, but that also the achievablerotational acceleration of the rotating element is increased and thecontrollability of the rotational speed and therewith, of the volumeflow can be improved.

Such a fluid pump can be integrated in the known way in a synchronousmotor without brushes. The fluid pump consists according to WO 00/640 30essentially of a cylindrical tube, which can be connected at both sidesto a fluid system. The tube is surrounded by the stator, consisting ofthe metal packet, the winding and the iron flux return hood. The rotorcomprises permanent magnetic field exciters and has on its outer coverdelivery devices for the fluid, so that the fluid can be axiallydelivered in the annular space between the tube and the rotor.

The rotor is magnetically supported. It carries for the purpose on itsboth end sides cylindrical or annular permanent magnets attachedthereon, which are magnetised in the axial direction. The permanentmagnets of the rotor are opposed by counter magnetised permanentmagnets, which e.g. can be arranged in the end sides of guiding devices,which themselves are mounted in the cylindrical tube.

Both magnet pairs act stabilisingly in radial direction, when they areorientated to attract each other, i.e., the radial support is passivelystable. The rotor is, however, instable in the axial direction.

Without additional stabilisation the rotor would be attracted by one ofthe two pairs of permanent magnets. Therefore, control coils arearranged on the stator sides in such a way, that a current weakens bythe in series connected control coils the magnetic field of one of thepairs of permanent magnets and increases the magnetic field of the otherpair of permanent magnets. The control current has to be adjusted independency of the actual axial rotor position. For this, the rotorposition has to be determined by means of position sensors.

The position sensors consist for example of two sensor coils, which canbe arranged on the end sides of the guiding devices. The sensor coilsare opposed on the ends of the rotor by aluminium bodies, in which eddycurrents are formed, when the sensor coils are loaded by an alternatingcurrent. By the axial movement of the rotor a change in the inductanceof the sensor coils is produced, which in an arrangement in a bridgeconnection can be evaluated as a measuring signal for the rotorposition.

As especially in a pulsating flow through the pump disturbing forces actcontinuously on the rotor, the position control has to be able toquickly adjust a changed axial rotor position. On the other hand thecontrol current should cause a low dissipation, which is especiallyimportant for blood pumps, as the produced heat energy should be kept assmall as possible. Furthermore, the drive energy has to be taken fromimplanted batteries, which operation time should be as long as possible.

SUMMARY OF THE INVENTION

An aspect of the invention is to provide a method for controlling theposition of a magnetically supported component, with which a dissipationof the control of the position can be kept small.

The aspect of the invention is described by the features of claim 1.Suitable embodiments are subject of the dependent claims.

According to this the current through the control current coils is pulsewidth modulated according to a desired value, to which a controllerarranged behind the position sensory analysis is set, wherein at a highdesired value a switching to a higher voltage level takes place. Thishas the advantage, that the adjustment times can be kept very small andthe necessary power can still be kept low.

The actual value of the position sensory analysis is stored for adefined time interval, respectively starting latest with the pulse edgeof the control current and the position sensory analysis is stoppedduring this time interval.

Advantageously in the use of the position control in a synchronous motorwithout brushes, the actual value of the position sensory analysis isalso temporarily stored in reference to the gatting impulse of the motorcoils for a defined time interval, starting latest with the pulse edgeof the gatting impulse, and the position sensory analysis is stopped forthis time interval.

The interferences produced by the timing concerning the positiondetermination are controlled by the stopping of the measuring duringthese time periods and by the storing of the measured values.

For specific applications it can be suitable, to take the square of thedesired value of the controller arranged behind the position sensoryanalysis mechanism and to stop the position adjustment at atime-averaged overshooting of the threshold of this value until the nextundershooting of the threshold. Thus, a rise in temperature of thecontrol coils is reproduced and therefore, an overheating is prevented.

Appropriately, a PID-controller with an I₂-component is used for thecontroller arranged behind the position sensory analysis mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail by means of an embodiment. In thecorresponding drawings:

FIG. 1 shows a sectional view through a fluid delivery pump, which issuitable for the execution of the method according to the invention;

FIG. 2 shows a representation of the principal of the position controlwith the additional flow control according to the invention; and

FIG. 3 shows a block circuit diagram of the position controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows such an axial pump suitable for the execution of themethod. The drive of the blood pump works according to the principal ofan electronic commutated synchronous motor. The motor has a stator,consisting of a metal sheet packet 31, of windings 33 and iron fluxreturn hoods 2, 2 a and a rotor 5 with a permanent magnetic core 32. Thestator encloses a tubular hollow body 1, in which in axial direction afluid, in the present case blood, is delivered. The rotor 5 is supportedmagnetically free of contact.

The magnetical support (bearing) consists of permanent magnets 42, 42 aon the rotor end sides and permanent magnets 41, 41 a on the end sidesof the guiding devices 6 and 7. The guiding devices 6, 7 are mounted onthe inner wall of the tubular hollow body 1.

To the magnetic support (bearing) further belong control coils 12, 12 a.Sensor coils 43, 43 a in the guiding devices 6, 7 and short circuitrings 80, 80 a arranged opposed thereto, serve for measuring the actualrotor position.

The pairs of permanent magnets 41, 42; 41 a, 42 a are, respectively,polarised for attracting each other. Magnetically the pairs are arrangedin series.

Without an additional stabilisation the rotor 5 would, however, beattracted to one side, therefore. An instable equilibrium exists inaxial direction. In radial direction both magnet pairs actself-centering, and, therefore, the radial position is passively stable.

The control coils 12, 12 a are connected electrically in series and aremagnetically arranged in such a way, that a current weakens the magneticfield of the one pair of magnets and increases the magnetic field of theother pair. The magnetic flux return path is produced via the iron fluxreturn hoods 2, 2 a and the metal sheet packet 31 of the stator.

The axial position of the rotor 5 can be determined by means of thesensor coils 43, 43 a. The sensor coils 43, 43 a are loaded by a higherfrequent voltage. By the axial movement of the rotor 5 a change of theinductivity of the sensor coils 43, 43 a is produced. By the arrangementof the sensor coils 43, 43 a in a bridge connection a measuring signalfor the axial position of the rotor 5 can be achieved.

As shown in FIG. 2, at the outlet of a controller, arranged behind theposition sensory analysis mechanism, an operating value of the controlcurrent is produced by the control coils 12, 12 a. The control currentis transmitted by means of a current controller to the control coils 12,12 a. The current controller acts as a closed control circle, i.e. itmeasures the current by means of the control coils 12, 12 a and comparesthe result with a default value (desired current) of the positioncontroller. By means of a pulse width modulation of a pulsed power stagethe actual current is adjusted to the desired current. This processnecessitates a specific time, which depends on the difference of thedesired current and the actual current. The higher the voltage is, withwhich the power stage operates, the shorter is the adjustment time ofthe current controller. On the other hand the dissipation in the powerstage increases with the voltage. To be able to achieve a quick reactionof the current controller and a lower dissipation, a higher voltage isadditionally switched on, only when a large difference between thedesired current and the actual current is present; otherwise it operateswith a lower voltage.

By the excitation of the control coils 12, 12 a by means of the pulsedpower stage interferences, which can disturb the position determinationof the rotor 5, are produced in the sensor coils 43, 43 a. Theseinterferences couple with each pulse edge on the control coils 12, 12 ato the sensor coils 43, 43 a and fade out after a defined time interval.Therefore, for the expected time interval of these interferences, theposition signal, achieved directly beforehand, is temporarily stored andthe position determination is stopped. The position controller worksduring this time interval with the stored value. When the interferenceis faded out, the position is again determined by means of the sensorcoils 43, 43 a. Similar interferences can also be produced by theexcitation of the windings 33. Also for these the method of thetemporary storing is used. The electronics of the interferencesuppression receive from the current controller and from the excitationelectronics of the motor the exact time of the possible starting pointof the interferences, so that they can store the position signals.

FIG. 3 shows the circuitry for the position control of the magneticsupport. From the measured position of the rotor, which loads the path21, a set current for the control coils 12, 12 a, which leads to asecure hovering of the rotor 5 in all operating conditions, isdetermined and transmitted to the outlet 22 of the position controller.The position controller consists of a PID-controller, which ischaracterised by the time constants of the integrator Ti and of thedifferentiator Td as well as by the multiplication factor kr of avariable gain amplifier. To protect the control coils 12, 12 a against athermal overload the to be expected dissipation is additionallydetermined from the current square. During a threshold overshooting timeaveraged over a low-pass, the position control is switched-off, untilthe threshold is again undershot. The position controller has as anadditional function, to keep the current through the control coils 12,12 a as low as possible. By means of an integrator (I₂-component) theset current is coupled back to the controller inlet. As a result therotor 5 is always positioned at the axial position in the pump, at whichonly a minimal current flows through the control coils 12, 12 a.

Reference numerals list

-   1 Tubular hollow body-   2 Iron flux return hood-   2 a Iron flux return hood-   5 Rotor-   6 Guiding device-   7 Guiding device-   12 Control coil-   12 a Control coil-   31 Metal sheet packet-   32 Permanent magnetic core-   33 Windings-   41 Permanent magnet-   41 a Permanent magnet-   42 Permanent magnet-   42 a Permanent magnet-   43 Sensor coil-   43 a Sensor coil-   21 Path-   22 Outlet-   80 Short circuit ring-   80 a Short circuit ring-   Ti Integrator-   Td Differentiator-   kr Multiplication factor

1. A method for controlling the position of a permanent magnetically supported rotating component, providing the steps of determining the position of the component through a position sensory analysis mechanism; providing control current coils for influencing the magnetic field of the permanent magnetic support; providing an adjustable current through the control current coils, the adjustable current being determined by the position of the component, wherein the current, through the control current coil, is pulse width modulated according to a predefined value, set in a controller coupled to the position sensory analysis mechanism, switching the adjustable current to a higher voltage stage when a higher desired value is required by the controller, and storing the actual value of the position sensory analysis for a defined time interval, starting respectively at the latest with the pulse edge of the control current, and stopping the position sensory analysis for the defined time interval.
 2. The method according to claim 1, wherein the rotating component is driven by a synchronous brushless motor having motor windings excitable by a pulse width modulated excitation current having pulse edges, the method further comprising the step of temporarily storing the actual value of the position sensory analysis for a second defined time interval, starting at the latest with the pulse edge of the excitation impulses and stopping the position sensory analysis for this seecond time interval.
 3. The method according to claim 1, further comprising a step of: providing a PID-controller with an I₂-component for the controller coupled to the position sensory analysis mechanism.
 4. A method for controlling the position of a permanent magnetically supported rotating component, providing the steps of providing a position sensor arrangement; providing control current coils capable of influencing the magnetic field of the permanent magnetic support; determining the current amount by the position of the rotating component, by adjusting the pulse width of the current through the control current coils, the pulse width being modulated according to a predefined value, set in a controller connected to the position sensory arrangement, and wherein at a high predefined value, switching the current to a higher voltage and storing the actual value of the position sensor arrangement for a defined time interval starting respectively at the latest with the pulse edge of the control current, and stopping the position sensory analysis for said defined time interval.
 5. A method for adjusting the position of a rotating component, which is supported by means of a permanent magnet and coils carrying a controlled current influencing the magnetic field of the permanent magnet, the method comprising the steps of: sensing the position of the rotating component with a position sensor, controlling the current delivered to the coils by varying the width of the current pulses based on the sensed position of the rotating component to achieve a target value; switching the current to a higher voltage if the target value is too high to be achieved by pulse width modulation; storing the sensed position of the rotating component for a defined period beginning, at the latest, with the pulse edge of the control current; and deactivating the position sensor during said defined period.
 6. The method of claim 1 or 5 further comprising the steps of: calculating the square of the target value specified by the controller connected to the output of the position sensor; deactivating the position sensor whenever the time averaged value of the square of the target value exceeds a threshold value; and maintaining the deactivation of the position sensor until the square of the target value drops below the threshold value. 